A technician uses ultrasound to inspect a sample made from a layer of gel bonded to a layer of polymer. A transmitter and detector are together at the front surface of the gel.
The sound has a frequency of 4.0 MHz. Each pulse of sound lasts 1.0 μs1.0\ \mu\text{s}1.0 μs.
| Material | Speed of sound in the material in m/s\text{m/s}m/s |
|---|---|
| gel | 1500 |
| polymer | 3000 |
The transmitter sends sound pulses perpendicular to the layers. Each reflected pulse returns along the same path.
| Echo | Reflecting surface | Time after transmission in μs\mu\text{s}μs |
|---|---|---|
| 1 | boundary between the gel and polymer | 12 |
| 2 | back surface of the polymer | 20 |
Calculate the number of complete oscillations in each pulse.
Calculate the wavelength of the sound in the gel.
Calculate the thickness of the polymer layer.
Explain what happens to the wavelength when the sound passes from the gel into the polymer.